Article pubs.acs.org/est
Insights on the Molecular Mechanism for the Recalcitrance of Biochars: Interactive Effects of Carbon and Silicon Components Jianhua Guo†,‡ and Baoliang Chen*,†,‡ †
Department of Environmental Science, Zhejiang University, Hangzhou 310058, China Zhejiang Provincial Key Laboratory of Organic Pollution Process and Control, Hangzhou 310058, China
‡
S Supporting Information *
ABSTRACT: Few studies have investigated the effects of structural heterogeneity (particularly the interactions of silicon and carbon) on the mechanisms for the recalcitrance of biochar. In this study, the molecular mechanisms for the recalcitrance of biochars derived from rice straw at 300, 500, and 700 °C (named RS300, RS500, and RS700, respectively) were elucidated. Short-term (24 h) and long-term (240 h) oxidation kinetics experiments were conducted under different concentrations of H2O2 to distinguish the stable carbon pools in the biochars. We discovered that the stabilities of the biochars were influenced not only by their aromaticity but also through possible protection by silicon encapsulation, which is regulated by pyrolysis temperatures. The aromatic components and recalcitrance of the biochars increased with increasing pyrolysis temperatures. The morphologies of the carbon forms in all of the biochars were also greatly associated with those of silica. Silica-encapsulation protection only occurred for RS500, not for RS300 and RS700. In RS300, carbon and silica were both amorphous, and they were easily decomposed by H2O2. The separation of crystalline silica from condensed aromatic carbon in RS700 eliminated the protective role of silicon on carbon. The effect of the biochar particle size on the stability of the biochar was greatly influenced by C−Si interactions and by the oxidation intensities. A novel silicon-and-carbon-coupled framework model was proposed to guide biochar carbon sequestration.
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example, 80−98 wt % rice hull ash silicas are amorphous,17 and they are always involved in the organic matrix mixture, forming a rigid microstructure that stabilizes the structures of plants18 and presumably is bound to carbohydrates.19 Silica ash can change the physical and structural compositions of biochar through pyrolysis via fusion and sintering.3,13,20 As previously reported, lower ash contents and higher temperatures will produce more highly aromatic biochars.11,21,22 Conversely, high ash contents may hinder the formation of aromatic carbon,21 because minerals in the precursor samples act as a barrier to the diffusion of heat and the release of degraded volatiles during the charring process.11 In addition, the fixed carbon contents were shown to decrease with increasing pyrolysis temperatures when biochars contained greater than 20% ash, suggesting possible interactions between organic and inorganic constituents during feedstock pyrolysis.21 Furthermore, the presence of significant amounts of minerals may cause defects in aromatic structures, thereby reducing the number of cross-links between layers and decreasing the overall stability of a structure that is dominated by C links.22 A wide distribution of silicon in the dry weight of biomass,23 ranging from 0.1% to 10%,24 generally causes silicon to accumulate in biochar during charring processes. With
INTRODUCTION Biochar is a carbon-rich residue that is formed during the pyrolysis of biomass, and it has been shown to be highly stable, persisting longer in soils than other forms of soil organic matter.1,2 Therefore, biochar amendment is considered to be an emerging tool for carbon sequestration.3 Biochar is characterized as a heterogeneous material with a range of partially to completely carbonized carbon forms,4−7 and it contains different forms of crystallized aromatic carbons and nonpolyaromatic carbons,8,9 as well as distinct ash contents in biochars derived from various feedstocks.10,11 Aromatic carbon typically has the greatest contribution to the recalcitrance of biochar.2 Understanding the long-term stability of biochar is hindered by its heterogeneous nature and particulate form.2,12,13 The oxidation of biochar typically begins on its surfaces. Therefore, quantifying the decomposition of fresh biochar may lead to an overestimation of its long-term decay.2,9 A recent report revealed that CO2 production primarily originated from polycyclic aromatic components rather than from any minor labile components for biochars that were produced at 650 and 700 °C by short-term lab incubation and with a predicted half-life of 67 years.9 Furthermore, if the halflifetime of biochar is extrapolated to more than 1000 years,14,15 polyaromatic carbons might not be the only determinant for the stabilities of biochars. Apart from polyaromatic carbons, silica in biomass primarily consists of amorphous phytoliths and opal A particles.16 For © 2014 American Chemical Society
Received: Revised: Accepted: Published: 9103
December 18, 2013 July 12, 2014 July 14, 2014 July 14, 2014 dx.doi.org/10.1021/es405647e | Environ. Sci. Technol. 2014, 48, 9103−9112
Environmental Science & Technology
Article
Chemical Oxidation Kinetics Experiment. The chemical oxidation of biochars by H2O2 is commonly used to evaluate their recalcitrance, which is relevant to the degradation of biochar under natural soil conditions.21,32−34 The concentration gradient of H2O2 in the oxidation experiments was regarded as the level of oxidation. Additionally, the stock concentration of H2O2 used in this study was 30% w/w, and it was diluted to different concentrations. The H2O2 oxidation experiment was conducted in a thermostatic water bath on the basis of a modification of a previously reported method.28 The oxidation of organic matter by hydrogen peroxide is influenced by many parameters (such as temperature, pH, and concentration), with temperature being a key parameter: a higher temperature can accelerate the reaction rate and shorten the process. Because hydrogen peroxide rapidly decomposes at temperatures greater than 70 °C,32 a suitable temperature of 60 °C was selected for the incubation experiment. Briefly, after 100 mg of biochar was added to a 50 mL plastic tube, 40.0 mL of H2O2 was slowly added to the tube and stirred with a vortex mixer for 1 min to ensure that the water and solid mixed well. The covered tube was placed in a thermostatic water bath, and the temperature was maintained at 60 ± 0.5 °C for 24 h for each treatment. During oxidation, the tube was repeatedly uncovered to release the air; the liquid was removed by filtration with a 0.45 μm membrane as soon as the experiment finished. The final oxidized residues were washed with deionized water and dried in an oven at 80 °C. The following three oxidation kinetics experiments were performed: short-term (24 h) oxidation, long-term (240 h) oxidation and effect of particle size. (1) For short-term oxidation, 16 concentrations of H2O2 (0.0%, 0.5%, 1.0%, 2.0%, 3.0%, 4.0%, 6.0%, 8.0%, 10.0%, 12.0%, 15.0%, 18.0%, 21.0%, 24.0%, 27.0% and 30.0% w/w) were selected to investigate carbon stability in biochars under different levels of oxidation. The selected samples of RS300, RS500, and RS700 with 60−100 mesh particle sizes were reacted with H2O2 solutions in a one-time 24 h treatment. (2) For long-term oxidation, three H2O2 concentrations (0.5%, 2.0%, and 8.0%) were selected as the different oxidation levels, and RS300, RS500, and RS700 samples with 60−100 mesh particle sizes were selected as the model biochars. The long-term experiments consisted of 10 successive 24 h treatments to probe the microstructures and chemical characteristics of the multilayered composition of biochar. (3) To investigate the effect of particle size on carbon loss under different degrees of oxidation, five particle size ranges (200 mesh) of RS300, RS500, and RS700 and three concentrations of H2O2 (0.5%, 2.0%, and 8.0%) were selected for the 240 h oxidation treatments. The carbon loss obtained from oxidation is defined as a negative increase from 0 to −100%. The carbon loss was calculated as follows:
increasing pyrolysis temperatures, the morphologies of the silicon components change from amorphous to crystalline,13,25−27 resulting in interactions between silicon and carbon.13 For example, a tiny amount of carbon from incompletely burned rice husk residue was not completely eliminated by higher and longer temperature treatments due to intertwined hydrated silica and lignocellulose.27 Silicon may play an important role in arranging the heterogeneous structural composition of carbon pools,13 including aromatic carbons, thereby having a considerable impact on carbon evolution when amended in soils. However, the roles of silicon components on the stabilities of biochars remain unknown. Thus, identifying the mechanisms for the recalcitrance of biochars and elucidating the interactive effects of silicon and carbon components are critical for determining the stabilities of biochars, which is the primary objective of this study. The relative stabilities of biochars were examined via their carbon loss through chemical oxidation. Acidic hydrogen peroxide was employed to evaluate the stabilities of heterogeneous carbon pools because H2O2 as an oxidizer can remove >95% of natural organic matter28 and decompose most refractory organic matter, especially aromatic carbons.29,30 Therefore, this compound might be a promising indicator for distinguishing the heterogeneity of the labile and stable components of biochar. Short-term (24 h) and long-term (240 h) oxidation kinetics experiments with a series of H2O2 concentrations were conducted to evaluate carbon stability in rice-straw-derived biochars. The impacts of the preparation temperature and particle size on the carbon loss of biochars were examined. The microstructures of the raw biochars and oxidized biochars were characterized using scanning electron microscopy with energy dispersive spectrometry (SEM-EDS) and Fourier transform infrared spectroscopy (FTIR).
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MATERIALS AND METHODS
Preparation of Biochars. Rice straw, one of the most abundant crop residues,31 was collected from Anhui Province, China, and air-dried and milled through 30, 60, 100, and 200 mesh sieves. Five different particle sizes (200 mesh) of rice straw were prepared as the biomass precursors. Biochar was produced by pyrolyzing the biomasses under oxygen-limiting conditions at 300, 500, and 700 °C using a previously reported method.6,7 Briefly, approximately 110 g of powdered rice straw was packed tightly in a ceramic crucible, which was then covered with a tightfitting lid and then pyrolyzed in a muffle furnace for 4 h. The charred samples were sieved through 30, 60, 100, and 200 mesh sieves to obtain the final biochar samples. Biochars for RS300 with different particle sizes were named RS300-()200. The symbol of ()200 corresponds to the particles sieved through a 200 mesh sieve, which is less that the particle size of 30−60, 60−100, and 100−200. The other particle sizes lie within the different meshes. For example, RS300−30−60 means RS300 with the particle passed through 30 mesh sieve but intercepted by 60 mesh sieve. The RS500 and RS700 samples were characterized using the same procedures. RS300, RS500, and RS700 refer to rice straw biochars produced at 300, 500, and 700 °C, respectively.
C loss =
C2 − C1 × 100% C1
(1)
Where Closs represents the carbon loss percentage, C1 refers to the carbon content in primary biochars, and C2 refers to the carbon content in oxidized biochar residues. Sample Characterization. The carbon contents (OC%) in the biochars and oxidized biochars were determined using a TOC-V carbon analysis instrument (SSM-5000A, Shimadzu, Japan). The ash contents (ash%) were measured by heating the 9104
dx.doi.org/10.1021/es405647e | Environ. Sci. Technol. 2014, 48, 9103−9112
Environmental Science & Technology
Article
Figure 1. SEM-EDS images of RS300 (A, B), RS500 (C, D) and RS700 (E, F). (B) One spot of agglomeration in RS300 that mainly consisted of K2O. (D) One of the crystals of potassium chlorite on the surface of RS500. (F) Shows the other surface of RS700 with accumulated silicate crystals. Note that the elemental components are the same color as the corresponding lines of the elements.
samples at 800 °C under an air atmosphere for 4 h. The organic carbon contents on an ash-free basis (OCash‑free%) were calculated as OCash‑free% = OC% / (100% − ash%). FTIR spectra were recorded in the 4000−400 cm−1 region with a resolution of 4 cm−1 using an FTIR spectrometer (Nicolet 6700, Thermo Scientific) and processed using the Thermo Scientific OMNIC software. SEM-EDS images of the samples were obtained by scanning the sample surfaces with a fieldemission scanning electron microscopy (Quanta 3D FEG, FEI). EDS line spectra were acquired by drawing ∼60 μm arrows at a 1000× magnification for 300 s to achieve a sufficient signal-tonoise ratio. The results were autocorrected and optimized using the Inca software. The specific surface areas of the prepared
biochars were measured by N2 (0.162 nm2) gas adsorption at liquid N2 temperature (−196 °C) using a NOVA-2000E surface area analyzer. Prior to the surface area measurements, the samples were degassed under vacuum (less than 0.1 Pa) for 12 h at 378 K. Four data points, between relative pressures of 0.05 and 0.3, were used to determine the monolayer adsorption capacity.
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RESULTS AND DISCUSSION Characterization of the Rice-Straw-Derived Biochars. The carbon and ash contents in the RS300, RS500, and RS700 samples with different particle sizes are listed in Table S-1 of 9105
dx.doi.org/10.1021/es405647e | Environ. Sci. Technol. 2014, 48, 9103−9112
Environmental Science & Technology
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formation of different types of aromatic carbon forms compared with RS300 and RS500. The peaks at 1100, 796, and 467 cm−1 were primarily assigned to Si−O−Si asymmetric vibrations, symmetrical stretching vibrations, and flexural vibrations, respectively.38 As previously reported, the C−O stretching vibration also appeared in the range of 1170−950 cm−1.13 Biochars prepared at 300 °C are generally composed of residues of biopolymers such as cellulose,6,7,40 so the possibility of C−O stretching cannot be excluded in RS300. The Si−O−Si peak at 1100− 1050 cm−1 was mainly assigned to the stretching of Si−O,41 which shifted from 1099 cm−1 for RS300 to 1091 cm−1 for RS500 and to 1056 cm−1 for RS700, indicating that the morphologies of silicon changed with increasing pyrolysis temperature. These results are supported by the SEM-EDS images shown in Figure 1. Effect of Short-Term H2O2 Oxidation on the Carbon Losses of Biochars. Biochars prepared at three different temperatures (i.e., RS300, RS500 and RS700) exhibited different carbon loss trends under different concentrations of H2O2 during short-term oxidation (Figure 2). RS300 exhibited
the Supporting Information. For all of the biochar samples, the carbon content on an ash-free basis decreased with decreasing particle size, and the ash content correspondingly increased. According to the elemental compositions presented in Table S2, RS700 was highly carbonized with polyaromatic carbon according to the measured O/Corg = 0.40 and H/Corg = 0.29. Comparatively, RS300 was the least carbonized biochar, with an elemental composition of O/Corg = 0.39 and H/Corg = 0.75, and RS500 had a considerably lower O/Corg = 0.33 but an intermediate H/Corg = 0.37, indicating that this sample was partially carbonized; these results are consistent with the previously reported NMR results33 and with the standards of the International Biochar Initiative, as H/Corg
